The heart, as is known, is a pump and, as such, produces pressure that causes the blood to circulate through the system. A widely used graphic representation of the cardiac function is known as the pressure-volume loop (PVL), referring to the relationship, in the left ventricle, between pressure and volume during a single cardiac cycle which consists of four main phases: (1) the heart being filled with blood; (2) the heart generating enough pressure to overcome arterial resistance; (3) the heart ejecting blood into the arteries, and (4) pressure in the heart dropping so that it is ready to receive blood again.
PVLs have a variety of uses. They have been found to reliably depict various external influences on the heart, such as exercise, drug therapy, cardiac disease, etc. PVLs are an irreplaceable research tool in the field of cardiac mechanics, and are extensively used in the teaching of medical students. Other parameters which can be determined with the aid of the method and system according to the present invention include cardiac power, cardiac peak power and contractile reserve.
Cardiac power, the power of the heart muscle, representing the pumping capability of the heart (in units of work/time), is a known index of the heart's pumping ability to increase power has been shown to be directly related to the survival of patients with severe heart failure [Tan, 1987]. Recent studies have shown the cardiac peak power, i.e., the maximum instantaneous cardiac power during blood ejection, to be an accurate descriptor of the heart's contractility, the quality of the heart muscle, representing its ability to contract [Kass and Beyar, 1991].
The present invention measures cardiac power as well as power increase under stress, thus calculating contractile reserve (CR), the difference between cardiac peak power and cardiac power at rest. CR can be used to arrive at a prognosis for heart failure patients, and thus to decide the timing of heart transplantation or other therapy. CR is an excellent tool for following the treatment of heart patients and for diagnosing heart disease.
Despite its many uses, no easy way has so far been found to produce PVLs. The only method of doing so until now has been by means of cardiac catheterization, which is an invasive procedure that, as all invasive procedures, puts the patient at a non-negligible risk, is very costly in that it requires a large, highly trained staff, and consequently is not performed in every hospital.